Rounded contact fingers on substrate/PCB for crack prevention

ABSTRACT

A strengthened semiconductor die substrate and package are disclosed. The substrate may include contact fingers formed with nonlinear edges. Providing a nonlinear contour to the contact finger edges reduces the mechanical stress exerted on the semiconductor die which would otherwise occur with straight edges to the contact fingers. The substrate may additionally or alternatively include plating traces extending at an angle from the contact fingers. Extending at an angle, at least the ends of the plating traces at the edge of the substrate are covered beneath a lid in which the semiconductor package is encased. Thus, when in use with a host device, contact between the ends of the plating traces beneath the lid and contact pins of the host device is avoided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate to a method of forming achip carrier substrate to alleviate chip and lead cracking, and a chipcarrier formed thereby.

2. Description of the Related Art

The strong growth in demand for portable consumer electronics is drivingthe need for high-capacity storage devices. Non-volatile semiconductormemory devices, such as flash memory storage cards, are becoming widelyused to meet the ever-growing demands on digital information storage andexchange. Their portability, versatility and rugged design, along withtheir high reliability and large capacity, have made such memory devicesideal for use in a wide variety of electronic devices, including forexample digital cameras, digital music players, video game consoles,PDAs and cellular telephones.

While a wide variety of packaging configurations are known, flash memorystorage cards may in general be fabricated as system-in-a-package (SiP)or multichip modules (MCM), where a plurality of die are mounted on asubstrate. The substrate may in general include a rigid base having aconductive layer etched on one or both sides. Electrical connections areformed between the die and the conductive layer(s), and the conductivelayer(s) provide an electric lead structure for integration of the dieinto an electronic system. Once electrical connections between the dieand substrate are made, the assembly is then typically encased in amolding compound to provide a protective package.

In view of the small form factor requirements, as well as the fact thatflash memory cards need to be removable and not permanently attached toa printed circuit board, such cards are often built of a land grid array(LGA) package. In an LGA package, the semiconductor die are electricallyconnected to exposed contact fingers formed on a lower surface of thepackage. External electrical connection with other electronic componentson a host printed circuit board (PCB) is accomplished by bringing thecontact fingers into pressure contact with complementary electrical padson the PCB. LGA packages are ideal for flash memory cards in that theyhave a smaller profile and lower inductance than pin grid array (PGA)and ball grid array (BGA) packages.

A cross-section of a conventional LGA package (without molding compound)is shown in FIG. 1. One or more die 20 are mounted on a substrate 22 viadie attach 24. The die are shown separated by a dielectric spacer layer26. In embodiments, the die 22 may be affixed to dielectric spacer layer26 by an epoxy. Generally, the substrate 22 is formed of a rigid core28, of for example polyimide laminate. Thin film copper layer(s) 30 maybe formed on the core in a desired electrical lead pattern, includingexposed surfaces for the contact fingers, using known photolithographyand etching processes. The contact fingers 32 may be formed of a layerof gold deposited on the copper layer 30 to provide the electricalconnection of the package to the host PCB. The substrate may be coatedwith a solder mask 36, leaving the contact fingers 32 exposed, toinsulate and protect the electrical lead pattern formed on thesubstrate. The solder mask covers the surfaces of the substrate, leavingexposed those areas that are to be soldered. The die may be electricallyconnected to the substrate by wire bonds 34. Vias 42 (FIG. 2) are formedthrough the substrate to allow electrical connection of the die throughthe substrate to the contact fingers 32. Further examples of typical LGApackages are disclosed in U.S. Pat. Nos. 4,684,184, 5,199,889 and5,232,372, which patents are incorporated by reference herein in theirentirety.

A bottom view of the substrate shown in prior art FIG. 1 is shown inprior art FIG. 2. As shown, the contact fingers 32 are generallyrectangular with leads 40 extending from respective fingers to vias 42or other electrical terminals. The contact fingers 32 also have platingtraces 46 connecting each of the fingers 32 to a plating bar (notshown). The plating bar connects all of the contact fingers to the sameelectrical potential for plating during an electroplating process. Inone such process, the substrate 22 is immersed in a plating bathincluding metal ions in an aqueous solution. A current is supplied tothe plating bar, which current travels through the plating traces 46 tocontact fingers 32. When the current is delivered, metal ions areattracted to the electrified and charged surfaces of the contactfingers. In this way, a layer of gold or other plating metal of adesired thickness may be deposited. After electroplating, the platingbar is removed leaving a portion of the plating traces 46 on thesubstrate 22.

After the die are mounted onto the substrate, the assembly is packagedwithin a molding compound to protect the assembly. During the moldingprocess, the molding machine may output an injection force typicallyabout 0.8 tons to drive the molding compound into the mold cavity. Fordie having a footprint of about 4.5mm by 2.5mm, this injection force mayresult in a pressure down on the die of about 1.2 kgf/mm².

A portion of the die 20 overlie an edge of the contact fingers. Theedges of all contact fingers define a straight line 54 beneath the die,where the height of the substrate is greater (owing to the contactfingers) on one side of line 54 than on the other side. Upon exertion ofthe molding force, the difference in height along straight line 54defined by the contact finger edges generates mechanical stress on thesemiconductor die.

In the past, semiconductor die were better able to withstand the stressgenerated during the molding process in LGA packages. However, chipscale packages (CSP) and the constant drive toward smaller form factorpackages require very thin die. It is presently known to employ waferbackgrind during the semiconductor fabrication process to thin die to arange of about 2 mils to 13 mils. At these thicknesses, the die areoften not able to withstand the stresses generated during the moldingprocess. Therefore, the die can crack, for example along the line 54.

Die cracking under the stress of the molding process will generallyresult in the package having to be discarded. Occurring at the end ofthe semiconductor fabrication and packaging process, this is anespecially costly and burdensome problem.

In addition to die cracking, the leads 40 connect to the contact fingers32 at right angles. The abrupt change in the conductance pattern at thejunction where a lead 40 connects to a contact finger 32 generatesmechanical stress at that junction.

Moreover, as shown in prior art FIG. 3, the solder mask 36 is appliedover the conductance pattern with openings for the contact fingers 32.The openings in the solder mask 36 at the contact fingers 32 begin atthe junctions between the leads 40 (beneath the solder mask) and thecontact fingers 32. Thus, the thickness of the substrate over the leads40 (including the solder mask) is greater than the thickness of thesubstrate over the fingers 32 (not having the solder mask). Thisdifference in thickness further generates mechanical stress at thejunction between the leads 40 and the contact fingers 32.

Thus, mechanical stress is generated at the junction between the leads40 and the contact fingers 32 both from the abrupt change in thepatterns at the junction and the differences in substrate thicknesses atthe junction. Over time, this mechanical stress can cause one or more ofthe leads 40 to break at or near the junction between the contactfingers and leads, resulting in package failure.

Separate and independent from the problem of die cracking and leadbreaking, the portions of plating traces 46 remaining after the platingtrace is severed from the contact fingers 32 tend to curl up over time.In particular, when a device formed from substrate 22 is used with ahost device, the substrate contact fingers 32 are brought into pressurecontact with pins on the host device. These host device pins often rubagainst the ends of plating traces 46. This contact may cause the endsof plating traces 46 to detach and curl up with repeated use of theflash memory device in a host device. While not a problem in and ofitself, this curling of the plating traces 46 can cause one or more ofthe metal layers on the contact fingers 32 to delaminate from thesubstrate. This delamination of the contact finger layers may result indamage to the package and/or package failure.

SUMMARY OF THE INVENTION

One embodiment relates to a strengthened semiconductor die substrate andpackage. The semiconductor die package is formed of one or moresemiconductor die mounted on a substrate, and a molding compound forencapsulating the one or more semiconductor die and substrate. Thesubstrate includes contact fingers for electrical connection of the diepackage to external components. In embodiments of the present invention,the contact fingers may include nonlinear edges. Providing a nonlinearcontour to the contact finger edges reduces the mechanical stressexerted on the semiconductor die which would otherwise occur withstraight edges to the contact fingers. In embodiments, the edges of thecontact fingers may have a rounded contour. In further embodiments, theedges of the contact fingers may have an irregular shaped contour.

In the embodiments, a taper may additionally be provided at the junctionbetween the rounded edges and electrical leads connecting the contactfingers to vias. The taper reduces stress the junction of the roundededge and electrical lead by distributing the forces at the junction overa larger area.

The edges of the contact fingers including the nonlinear contour mayvary in alternative embodiments. Namely, a contact finger may have fouredges: a first edge proximal to the near edge of the substrate, a secondedge opposite the first edge, and third and fourth edges extendingbetween and connecting the first and second edges. In embodiments, anyone or more of the first, second, third and/or fourth edges of a contactfinger may have a nonlinear contour.

Embodiments may additionally or alternatively include plating tracesextending at an oblique angle from the top, corner or side of thecontact fingers so as to terminate at an edge of the substrate at alocation between adjacent contact fingers. In this position, at leastthe ends of the plating traces at the edge of the substrate are coveredbeneath a lid in which the finished semiconductor package is encased. Byproviding the plating traces angled off to the side of the contactfingers with ends beneath the lid, contact between the pins on a hostdevice and the ends of the plating traces is avoided when the finishedsemiconductor device is used with the host device. Thus, the platingtraces do not detach from the substrate and the contact fingers remainsolidly laminated on the substrate.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional side view of a portion of a semiconductorpackage including semiconductor die mounted on a conventional substrate.

FIG. 2 is a bottom view of a conventional substrate including aplurality of contact fingers and other electrical contacts.

FIG. 3 is a bottom view of a conventional substrate shown in FIG. 2,including a layer of solder mask.

FIG. 4 is a cross sectional side view of a portion of a semiconductorpackage, including semiconductor die mounted on a substrate according toembodiments of the present invention.

FIG. 5 is a cross sectional side view of a portion of a semiconductorpackage, including an alternative configuration of semiconductor die ona substrate according to embodiments of the present invention.

FIG. 6 is a bottom view of a substrate according to embodiments of thepresent invention, including contact fingers with rounded edges.

FIG. 7 is an enlarged partial view of the substrate shown in FIG. 6.

FIG. 8 is a flowchart of a process for forming substrates according toembodiments of the present invention.

FIG. 9 is a bottom view of a substrate, including contact fingersaccording to an alternative embodiment of the present invention.

FIG. 10 is an enlarged view of a portion of the substrate shown in FIG.9.

FIG. 11 is a bottom view of a substrate, including contact fingersaccording to a further alternative embodiment of the present invention.

FIG. 12 is an enlarged view of a portion of the substrate shown in FIG.11.

FIG. 13 is a bottom view of a substrate, including contact fingersaccording to a further alternative embodiment of the present invention.

FIG. 14 is a bottom view of a substrate according to embodiments of thepresent invention, including a layer of solder mask.

FIG. 15 is a bottom view of a substrate according to embodiments of thepresent invention, including an alternative application of solder mask.

FIG. 16 is a cross sectional side view of a semiconductor packageaccording to embodiments of the present invention.

FIG. 17 is a bottom view of a flash memory device including asemiconductor package according to the embodiments of the presentinvention encased within a lid.

FIG. 18 is a bottom view of a flash memory device including platingtraces angled off to the side according to embodiments of the presentinvention.

FIG. 19 is an enlarged view of a portion of the flash memory device ofFIG. 18 showing the angled plating traces having portions covered by alid.

FIG. 20 is an enlarged view of a portion of the flash memory device ofFIG. 18 showing the angled plating traces and contact pins of a hostelectronic device.

FIG. 21 is a flowchart of a process for forming a semiconductor packageaccording to embodiments of the present invention.

DETAILED DESCRIPTION

Embodiments will now be described with reference to FIGS. 4 through 21,which relate to a strengthened semiconductor package. It is understoodthat the present invention may be embodied in many different forms andshould not be construed as being limited to the embodiments set forthherein. Rather, these embodiments are provided so that this disclosurewill be thorough and complete and will fully convey the invention tothose skilled in the art. Indeed, the invention is intended to coveralternatives, modifications and equivalents of these embodiments, whichare included within the scope and spirit of the invention as defined bythe appended claims. Furthermore, in the following detailed descriptionof the present invention, numerous specific details are set forth inorder to provide a thorough understanding of the present invention.However, it will be clear to those of ordinary skill in the art that thepresent invention may be practiced without such specific details.

FIGS. 4 and 5 are cross-sectional side views of a substrate 100 havingalternative configurations of stacked semiconductor die 116. Althoughnot clearly seen in the cross-sectional view of FIGS. 4 and 5, substrate100 differs from prior art substrates such as that shown in FIG. 1 atleast in that substrate 100 includes contact fingers with rounded orotherwise nonlinear edges as explained hereinafter. FIG. 6 and theenlarged view of FIG. 7 are partial bottom views of a portion of thesubstrate 100 shown in FIGS. 4 and/or 5 and having an edge 101.

Substrate 100 may be a variety of different chip carrier mediums,including a PCB, a leadframe or a tape automated bonded (TAB) tape.Where substrate 100 is PCB, the substrate may be formed of a core 106,having a top conductive layer 108 formed on a top surface of the core106, and a bottom conductive layer 110 formed on the bottom surface ofthe core. The core 106 may be formed of various dielectric materialssuch as for example, polyimide laminates, epoxy resins including FR4 andFR5, bismaleimide triazine (BT), and the like. Although not critical tothe present invention, core 106 may have a thickness of between 40microns (μm) to 200 μm, although the thickness of the core may varyoutside of that range in alternative embodiments. The core 106 may beceramic or organic in alternative embodiments.

The conductive layers 108 and 110 may be formed of copper or copperalloys, plated copper or plated copper alloys, Alloy 42 (42Fe/58Ni),copper plated steel, or other metals and materials known for use onsubstrates. The layers 108 and 110 may have a thickness of about 10 μmto 24 μm, although the thickness of the layers 108 and 110 may varyoutside of that range in alternative embodiments.

The layer 108 and/or layer 110 may be etched with a conductance patternfor communicating signals between one or more semiconductor die and anexternal device. One process for forming the conductance pattern on thesubstrate 100 is explained with reference to the flowchart of FIG. 8.The surfaces of conductive layers 108 and 110 are cleaned in step 250. Aphotoresist film is then applied over the surfaces of layers 108 and 110in step 252. A pattern mask containing the outline of the electricalconductance pattern may then be placed over the photoresist film in step254. The photoresist film is exposed (step 256) and developed (step 258)to remove the photoresist from areas on the conductive layers that areto be etched. The exposed areas are next etched away using an etchantsuch as ferric chloride in step 260 to define the conductance patternson the core. Next, the photoresist is removed in step 262. Other knownmethods for forming the conductance pattern on substrate 100 arecontemplated.

Once patterned, the top and bottom conductive layers 108, 110 may belaminated with a solder mask 112 in a step 264. In embodiments wheresubstrate 100 is used for example as an LGA package, one or more goldlayers may be formed on portions of the bottom conductive layer 110 instep 266 to define contact fingers 114 as is known in the art forcommunication with external devices. The one or more gold layers may beapplied in a known electroplating process. It is understood that thepresent invention may be used with other types of semiconductorpackages, including for example BGA packages.

FIG. 4 further shows two stacked semiconductor die 116 mounted on thesubstrate 100. The die 116 are shown separated by a silicon spacer 118as is known in the art. FIG. 5 illustrates a portion of an alternativesemiconductor package including a pair of semiconductor die 116 mounteddirectly adjacent to each other. The top semiconductor die 116 issmaller than the bottom semiconductor die 116 in the embodiment of FIG.5. Thus, electrical leads may be connected to the bottom semiconductordie at the edges of the bottom semiconductor die extending beyond theedges of the top semiconductor die. Embodiments of the invention mayalternatively include a single die, and embodiments of the invention mayalternatively include between 3 and 8 or more die stacked in an SiP, MCMor other type of arrangement. The one or more die may have thicknessesranging between 2 mils to 20 mils, but the one or more die may bethinner than 2 mils and thicker than 20 mils in alternative embodiments.While not critical to the present invention, the one or more die 116 maybe a flash memory chip (NOR/NAND), SRAM or DDT, and/or a controller chipsuch as an ASIC. Other silicon chips are contemplated.

The one or more die 116 may be mounted on the top surface of thesubstrate 100 using a known adhesive or eutectic die bond process, witha known die attach compound 120. Such die attach compounds aremanufactured for example by Semiconductor Packaging Materials, Inc. ofArmonk, N.Y. The one or more die 116 in FIGS. 4 and 5 may beelectrically connected to conductive layers 108, 110 of the substrate100 by wire bonds 122 using a known wire bond process.

FIG. 6 shows a bottom view of the positions of contact fingers 114formed on the bottom surface of the substrate 100. The contact fingers114 are provided to establish an electrical connection in a known mannerwith contact pins of a host PCB (not shown), or other electroniccomponent, when the contact fingers 114 are brought into pressurecontact against the contact pins (or other connectors) of the host PCB.While ten contact fingers 114 are shown, it is understood that there maybe more or less than ten contact fingers in alternative configurationsof the chip carrier substrate 100. In an embodiment, there may be eightcontact fingers. The contact fingers 114 are shown connected to vias 124by leads 126 defined in the conductance pattern in layer 110. Vias 124are provided to communicate electrical signals between the top andbottom conductance patterns on layers 108 and 110.

Referring to the bottom view of FIG. 6 and the enlarged view of FIG. 7,in embodiments of the present invention, contact fingers 114 may includerounded edges 130 at an end of contact fingers 114 adjacent toelectrical leads 126. As explained hereinafter, an opposite end 132 ofcontact fingers 114 may additionally include rounded edges. The roundededges 130 may be defined in the pattern mask and formed integrally withcontact fingers 114 during the photolithography process as describedabove for forming the conductance pattern(s) on the substrate 100. Inthe embodiments, the radius of curvature of rounded edges 130 may be onehalf the diameter, d, of contact fingers 114 so as to form half circlesat the end of the contact fingers. However, as will be explainedhereinafter, the radius of curvature of rounded edges 130 may vary inalternative embodiments of the present invention.

As explained in the background of the invention, conventional contactfingers define a straight edged stepped surface beneath thesemiconductor die. A downward force on the die lying atop such astraight edged stepped surface generates much larger mechanical stressin the die than would the same downward force on the die lying atopcurved edges. Rounded or nonlinear shaped edges provide support in twodimensions in the plane of the die, where a straight edge does not.Providing a rounded shape to the contact finger edges 130 removes thestraight edge otherwise defined by the collective ends of conventionalcontact fingers. Thus, the rounded edges 130 in embodiments of thepresent invention reduce the mechanical stress exerted on thesemiconductor die 116.

Referring to FIG. 7, embodiments of the present invention may include ataper 136 at the junction between the electrical leads 126 and therounded edges 130 of each of the contact fingers 114. As described inthe Background of the Invention section, the abrupt change where anelectrical lead connects to a contact finger often generates largemechanical stress at that junction. Taper 136 serves to alleviate themechanical stress at the junction by distributing the forces at thejunction over a larger area.

Tapers 136 are defined in the conductance pattern during thephotolithography process. The amount of metal left on the substrate todefine each of the tapers 136 may vary in alternative embodiments. Forexample, tapers 136 may begin with a width equal to electrical lead 126and widen out to a width between two and twenty times greater than thewidth of electrical lead 126, and more particularly between two and tentimes greater than the width of the electrical lead, at the junctionwhere the tapers 136 connect with rounded edges 130. The width of ataper 136 at the junction where the taper connects with a rounded edgemay be less than twice the width of the electrical lead 126 and morethan twenty times the width of the electrical lead 126 in alternativeembodiments.

The tapers 136 may be generally triangular in shape, and may havestraight, concave or convex sides extending from electrical leads 126 tothe junction with rounded edges 130. As seen in FIGS. 6 and 7, tapers136 may be formed around the radius of rounded edge 130 at any location,depending on where an electrical lead 126 joins to the rounded edge 130.Moreover, the shape of tapers 136 may vary depending on the anglebetween electrical lead 126 and rounded edge 130. For example, FIG. 7shows contact fingers 114 a and 114 b, which are examples of contactfingers 114. Electrical lead 126 meets contact finger 114 a at anoblique angle, where electrical lead 126 meets contact finger 114 b at asubstantially right angle. The shape of the tapers 136 may thus bedifferent at contact finger 114 a than 114 b.

While all junctions between electrical leads 126 and contact fingers 114may include a taper 136, taper 136 may be omitted from some or all ofthe junctions between leads 126 and contact fingers 114 in theembodiments of the present invention. Moreover, in embodiments, theshape, width and/or length of tapers 136 may be different for eachcontact finger 114, or different for each of two or more subsets ofcontact fingers 114.

Referring now to FIGS. 9 and 10, as opposed to a rounded edge 130, othernonlinear configurations may be provided to the edge of the contactfingers 114 to prevent a straight line being defined by the edges of thecontact fingers 114. For example, in FIGS. 9 and 10, contact fingers 114include irregular shaped edges 140. Each of the edges 140 may includethe same irregular shape, or the irregular shape of each of the edges140 may vary relative to each other. The irregular shape of edge 140 maybe defined in the pattern mask during the photolithography process. Inthe embodiments, a taper 136 as described above may be provided at eachconnection point between electrical lead 126 and irregular shaped edge140. Taper 136 may be omitted at the connection point between theelectrical leads and some or all of the contact finger edges 140 in theembodiments of the present invention.

In the embodiments described above, each contact finger 114 has anonlinear lower edge (i.e., an edge of fingers 114 distal from edge 101of substrate 100). In alternative embodiments, it is understood that oneor more of the contact fingers 114 may have a linear lower edge, but thelower edges of all of the contact fingers taken together define anonlinear contour across the substrate. As used herein, “nonlinear”refers to a line, profile or contour that is not straight.

In FIGS. 6, 7, 9 and 10, a taper 136 may be provided to reduce themechanical stress at the connection point of the electrical lead to thecontact finger 114. In a further embodiment of the present invention,the electrical lead 126 may be omitted altogether, and the contactfinger 114 itself may extend into direct electrical contact with the via124. For example, as shown in FIGS. 11 and 12, contact fingers 114 a,114 b, and 114 c have edges 144 which extend in direct electricalcontact with vias 124. In FIGS. 11 through 13, contact fingers 114 a,114 b, 114 c, 114 d, 114 e and 114 f are examples of contact fingers114. Edges 144 of contact fingers 114 a, 114 b and 114 c may tapersmoothly to vias 124, as in fingers 114 a and 114 c. Alternatively, edge144 may have an irregular taper to via 124, as shown in finger 114 b.The shape of edge 144 may be defined in the photo mask during thephotolithography process. While only three fingers 114 a, 114 b, and 114c are shown with a lower edge extending directly to vias 124, it isunderstood that all of the fingers may have lower edges 144 extending totheir associated vias.

As described above, where a contact finger 114 has a rounded lower edge,the radius of curvature of that lower edge may vary with respect to therounded edges of other contact fingers 114 in alternative embodiments.FIG. 11 shows a finger 114 e having a radius of curvature of its loweredge 130 which may be for example a half circle as described above.However, fingers 114 d and 114 f have lower edges 130 with larger radiiof curvature, therefore extending downward a shorter distance thanrounded edge 130 on contact finger 114 e. Different contact fingers 114may have different radii of curvature. One reason for having a largerradius of curvature is because a via 124 is located close to the bottomedge of contact finger 114 and a smaller radius of curvature is notpractical.

As indicated above, a second edge 132 of contact fingers 114 may alsohave a rounded edge. Such embodiment is shown in FIG. 13. Edges 132 mayhave differing radii of curvature as described above. Moreover, insteadof it being rounded, edges 132 may have an irregular shape as describedabove with respect to FIGS. 7 and 8. Less than all of the contactfingers 114 may include an upper rounded edge 132 in alternativeembodiments of the present invention. While lower rounded edges 130 andupper rounded edges 132 are shown as being concave, it is understoodthat edges 130 and/or 132 may be convex in alternative embodiments ofthe present invention. In a further embodiment of the present invention,it is understood that the one or both of the two sides of contactfingers 114 extending between and connecting edges 130 and 132 may havenonlinear edges in accordance with any of the embodiments of edges 130and 132 described herein.

Referring now to FIG. 14, after formation of the conductance pattern ofsubstrate 100 and plating of fingers 114 and other electrical leads, thesolder mask 112 may be applied to the substrate. As shown in FIG. 12,solder mask 112 may cover the lower rounded edges and (if present), theupper rounded edges of contact fingers 114. Alternatively, the soldermask 112 may be applied so as not to cover the lower rounded edge and/orthe upper rounded edge in alternative embodiments of the presentinvention, such as shown in FIG. 15. While rounded edges are shown inFIGS. 14 and 15, it is understood that solder mask 112 may be applied soas to cover or not cover any configuration of the nonlinear edges ofcontact fingers 114 described above.

FIG. 16 is a cross-sectional side view of a finished semiconductor diepackage 160 having substrate 100 and die 116 encased within moldingcompound 150. After the solder mask 112 is applied, and the wire bondconnections are made, the substrate 100 and die 116 may be encasedwithin the molding compound 150 in a known encapsulation process to forma finished semiconductor die package 160. Molding compound 150 may be anepoxy such as for example available from Sumitomo Corp. and Nitto DenkoCorp., both having headquarters in Japan. Other molding compounds fromother manufacturers are contemplated. The molding compound may beapplied according to various processes, including by transfer molding orinjection molding techniques, to encapsulate the substrate 100 andsemiconductor die 116.

FIG. 17 illustrates package 160 enclosed within a lid 170 to define aflash memory device 180. The lid 170 shown in FIG. 17 covers thenonlinear edges of contact fingers 114. Such a lid may be used in anembodiment where the solder mask also covers the nonlinear edges (FIG.14) or the solder mask does not cover the nonlinear edges (FIG. 15). Ina further embodiment, the lid may have curved or other non-linear edgesso as to expose the non-linear edges of the contact fingers. It isunderstood that flash memory device 180 may be any of various devicesincluding contact fingers 114 for mating with contact pins on a hostdevice. It is further understood that package 160 may be used as a flashmemory device 180 without being encased within a lid 170 in embodiments.

Referring to FIGS. 18 through 20, embodiments of the present inventionmay include plating traces 190 used during the electroplating process ofcontact fingers 114. Plating traces 190 may extend at an oblique anglefrom the top, corner or side of the contact fingers so as to terminateat an edge of the substrate at a location between adjacent contactfingers 114. As explained in the Background of the Invention section,conventional plating traces extended straight up from the contact pins.Thus, when the contact fingers were brought into pressure contact withthe host device, pins on the host device often rubbed against theplating traces. This frictional contact may cause the plating traces todetach from the substrate, which could result in delamination of thecontact finger from the substrate.

As best seen in FIG. 18, each plating trace may include a first section190 a extending at a 45° angle from the top or corner of a contactfinger 114, and a second section 190 b extending upward from the firstsection 190 a. It is understood that the first section may extend atangles greater than or less than 45° in alternative embodiments. Thesecond section 190 b lies between adjacent contact fingers 114, and mayconnect to a plating bar (not shown) during the fabrication process. Theconnection is severed and the plating bar is removed after the platingprocess is complete.

By providing plating traces 190 angled off to the side of the contactfingers 114, the sections 190 b and a portion of sections 190 a liebeneath ribs 172 of lid 170. Thus, when flash memory device 180 isbrought into contact with a host device having pins 192 (shown inphantom in FIG. 20), the pins 192 engage contact fingers 114 withouttouching the portions of the plating traces beneath the ribs 172. Thus,the plating traces do not detach from the substrate 100 and the contactfingers 114 remain solidly laminated on the substrate. Angled platingtraces 190 may be defined in the photo mask during the photolithographyprocess and plated during the electroplating process of contact fingers114. Angled plating traces 190 may be used in the embodiments of a flashmemory device including lids 170, or where package 160 operates withouta lid 170.

The flowchart of FIG. 21 sets forth an overall process for forming afinished die package from a starting point of a large substrate panel.As explained hereinafter, the substrate panel is separated intoindividual substrates after fabrication. In a step 270, the panel isdrilled to provide reference holes off of which the position of therespective substrates is defined. The conductance pattern is then formedon the respective surfaces of the panel in step 272 as explained above.The patterned panel is then inspected in an automatic optical inspection(AOI) in step 274. Once inspected, the solder mask is applied to thepanel in step 276.

After the solder mask is applied, the contact fingers may be plated. Asoft gold layer is applied over certain exposed surfaces of theconductive layer on the bottom surface of the substrate, as for exampleby thin film deposition, in step 278. As the contact fingers are subjectto wear by contact with external electrical connections, a hard layer ofgold may be applied, as for example by electroplating, in step 280. Itis understood that a single layer of gold may be applied in alternativeembodiments.

The individual substrates are then inspected and tested in an automatedstep (step 282) and in a final visual inspection (step 284) to checkelectrical operation, and for contamination, scratches anddiscoloration. The substrates that pass inspection are then sent throughthe die attach process in step 286. The wire bonds and other electricalconnections are then made on the substrate in a step 288, and thesubstrate and die are then packaged in step 290 in a known injectionmold process to form a JEDEC standard (or other) package as describedabove.

A cutting device then separates the panel into individual packages 160in step 292. Where the packages 160 form a flash memory within lids 170,the packages may be enclosed within lids 170 in a step 294. It isunderstood that a die package 160 including nonlinear edged contactfingers may be formed by other processes in alternative embodiments.

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A substrate for a semiconductor package, the substrate comprising:one or more contact fingers, formed on the substrate, for establishingelectrical communication between the semiconductor package and anexternal device, the one or more contact fingers having a nonlinearedge.
 2. A substrate as recited in claim 1, a contact finger of the oneor more contact fingers including four edges, a first edge proximate toa substrate edge, a second edge opposite the first edge and distal fromthe substrate edge, and third and fourth edges extending between andconnecting the first and second edges, the second edge of the contactfinger being nonlinear.
 3. A substrate as recited in claim 1, a contactfinger of the one or more contact fingers including four edges, a firstedge proximate to a substrate edge, a second edge opposite the firstedge and distal from the substrate edge, and third and fourth edgesextending between and connecting the first and second edges, the firstedge of the contact finger being nonlinear.
 4. A substrate as recited inclaim 1, a contact finger of the one or more contact fingers includingfour edges, a first edge proximate to a substrate edge, a second edgeopposite the first edge and distal from the substrate edge, and thirdand fourth edges extending between and connecting the first and secondedges, at least one the third and forth edges of the contact fingerbeing nonlinear.
 5. A substrate as recited in claim 1, the nonlinearedge being rounded.
 6. A substrate as recited in claim 1, the nonlinearedge having an irregular shaped contour.
 7. A substrate as recited inclaim 1, further comprising a via through the substrate and anelectrical lead on the substrate, the electrical lead connecting to acontact finger of the one or more contact fingers at the nonlinear edgeof the contact finger, and the electrical lead electrically connectingthe contact finger to the via.
 8. A substrate as recited in claim 7,further comprising a taper at a junction between the nonlinear edge ofthe contact finger and the electrical lead.
 9. A substrate as recited inclaim 1, further comprising a via through the substrate, a contactfinger of the one or more contact fingers extending into direct contactwith the via.
 10. A substrate as recited in claim 1, further comprisinga solder mask covering the nonlinear edge of the one or more contactfingers.
 11. A substrate as recited in claim 1, further comprising asolder mask not covering the nonlinear edge of the one or more contactfingers.
 12. A substrate for a semiconductor package, the substratecomprising: a plurality of contact fingers, formed on the substrate, forestablishing electrical communication between the semiconductor packageand an external device, at least one contact finger of the plurality ofcontact fingers having a rounded edge in electrical communication withan electrical lead formed on the substrate.
 13. A substrate as recitedin claim 12, a taper being provided at a junction between the roundededge and the electrical lead.
 14. A substrate as recited in claim 12,wherein the substrate is a printed circuit board.
 15. A substrate asrecited in claim 12, wherein the substrate is a leadframe.
 16. Asubstrate as recited in claim 12, wherein the substrate is a tape usedin a tape automated bonding process.
 17. A substrate as recited in claim12, further comprising a via through the substrate and an electricallead on the substrate, the electrical lead connecting to the roundededge of the at least one contact finger, and the electrical leadelectrically connecting the at least one contact finger to the via. 18.A substrate as recited in claim 12, further comprising a via through thesubstrate, the at least one contact finger extending into direct contactwith the via.
 19. A substrate for a semiconductor package, the substratehaving a first edge, the substrate comprising: a plurality of contactfingers, formed on the substrate, for establishing electricalcommunication between the semiconductor package and an external device,each contact finger of the plurality of contact fingers including afirst end proximal to the first edge of the substrate and a second end,opposite the first end and distal from the first edge of the substrate,the second end of each contact finger of the plurality of contactfingers together defining a nonlinear contour across the substrate. 20.A substrate as recited in claim 19, wherein the second end of eachcontact finger has a nonlinear contour.
 21. A substrate as recited inclaim 20, wherein the nonlinear contour of the second end of eachcontact finger is rounded.
 22. A substrate as recited in claim 20,wherein the nonlinear contour of the second end of each contact fingeris irregularly shaped.
 23. A flash memory device, comprising: asemiconductor package including a substrate and at least onesemiconductor die, the substrate including one or more contact fingersfor establishing electrical communication between the flash memorydevice and an external device, the one or more contact fingers having anonlinear edge.
 24. A flash memory device as recited in claim 23, the atleast one semiconductor die including two semiconductor die spaced fromeach other by a silicon spacer.
 25. A flash memory device as recited inclaim 23, the at least one semiconductor die including a firstsemiconductor die mounted directly atop a second semiconductor die. 26.A flash memory device as recited in claim 23, the semiconductor packagecomprising a land grid array package.
 27. A flash memory device asrecited in claim 23, the semiconductor package comprising a ball gridarray package.
 28. A flash memory device as recited in claim 23, acontact finger of the one or more contact fingers including four edges,a first edge proximate to a substrate edge, a second edge opposite thefirst edge and distal from the substrate edge, and third and fourthedges extending between and connecting the first and second edges, thesecond edge of the contact finger being nonlinear.
 29. A flash memorydevice as recited in claim 23, a contact finger of the one or morecontact fingers including four edges, a first edge proximate to asubstrate edge, a second edge opposite the first edge and distal fromthe substrate edge, and third and fourth edges extending between andconnecting the first and second edges, the first edge of the contactfinger being nonlinear.
 30. A flash memory device as recited in claim23, the nonlinear edge being rounded.
 31. A flash memory device asrecited in claim 23, the nonlinear edge having an irregular shapedcontour.
 32. A flash memory device as recited in claim 23, furthercomprising a lid for encasing the semiconductor package, the lidcovering the nonlinear edge of the one or more contact fingers.
 33. Aflash memory device as recited in claim 23, further comprising a lid forencasing the semiconductor package, the lid not covering the nonlinearedge of the one or more contact fingers.
 34. A method of reducing stresswithin semiconductor die in a semiconductor package, comprising thesteps of: (a) defining contact fingers on a substrate within thesemiconductor package, the contact fingers being defined with nonlinearedges; and (b) mounting the semiconductor die to the substrate over thenonlinear edges defines in said step (a).
 35. A method as recited inclaim 34, said step (a) comprising the step of defining the shape of thecontact fingers including the nonlinear edges in a mask pattern for aphotolithography process.
 36. A method as recited in claim 34, said step(a) comprising the step of electroplating the contact fingers.